1. Field of the Invention
The present invention relates to a method for producing L-cysteine. More precisely, it relates to a microorganism suitable for the production of L-cysteine and a method for producing L-cysteine utilizing such a microorganism. L-Cysteine and L-cysteine derivatives are used in the fields of drugs, cosmetics and foods.
2. Related Art
L-Cysteine is conventionally obtained by extracting it from keratin-containing substances such as hairs, horns and feathers or conversion of DL-2-aminothiazoline-4-carboxylic acid as a precursor using a microbial enzyme. It is also planned to produce L-cysteine in a large scale by an immobilized enzyme method utilizing a novel enzyme.
Furthermore, it is also attempted to produce L-cysteine by fermentation utilizing a microorganism. For example, the inventors of the present invention disclosed a method for producing L-cysteine by using a bacterium belonging to the genus Escherichia having serine acetyltransferase (EC 2.3.1.30, also referred to as “SAT” hereinafter) in which L-cysteine decomposition system is suppressed and feedback inhibition by L-cysteine is reduced (Japanese Patent Laid-open Publication (Kokai) No. 11-155571). As means for suppressing the L-cysteine decomposition system, reduction of intracellular cysteine desulfhydrase (also referred to as “CD” herein after) activity is disclosed. There is also known a method for producing L-cysteine by using a microorganism of which cysteine metabolism is decontrolled by using a DNA sequence coding for SAT that has a specific mutation so as to reduce feedback inhibition by L-cysteine (International Patent Publication in Japanese (Kohyo) No. 2000-504926).
Further, FEMS Microbiol. Lett., 179, 453-459 (1999) discloses a method for producing L-cysteine by using Escherichia coli introduced with a gene coding for an SAT isozyme derived from Arabidopsis thaliana, which does not suffer from feedback inhibition by L-cysteine.
Moreover, Japanese Patent Laid-open Publication No. 11-56381 discloses a method for producing L-cysteine using a microorganism overexpressing a gene coding for a protein suitable for releasing an antibiotic or a substance toxic to a microorganism directly from a cell.
As described above, various researches have been made about enhancement of activities of L-cysteine biosynthesis enzymes such as SAT and L-cysteine producing bacteria of which L-cysteine excretory system is modified. However, the L-cysteine decomposition system has not been studied in detail, in particular, for bacteria belonging to the genus Escherichia. 
As enzymes somewhat showing CD activity in Escherichia coli, there have been reported cystathionine-β-lyase (metC product, also referred to as “CBL” hereinafter, Chandra et al., Biochemistry, 21, 3064-3069 (1982)) and tryptophanase (tnaA product, also referred to as “TNase”, Austin Newton et al., J. Biol. Chem., 240, 1211-1218 (1965)). However, it is considered that CBL and TNase are enzymes catalyzing a reaction of converting cystathionine into homocysteine and a reaction of decomposing tryptophan, respectively, and it is not known whether they are enzymes substantially involved in the L-cysteine decomposition system. Further, although a mutant strain showing reduced CD activity is disclosed in Japanese Patent Laid-open Publication No. 11-155571, it does not report identity of the enzyme responsible for the CD activity.